Conventionally, the mainstream of a position at which a head within a hard disk drive is put on standby has been the upper part of the inner radius of a magnetic disk (CSS system). This system has a problem as described hereinafter.
That is, in order to improve the magnetic recording density of a hard disk, it has recently been required to make the flying height of a head lower so as to decrease the gap size between the head and the disk. In order to satisfy such requirements, it is necessary to improve the surface smoothness of each of the head slider and the disk. However, there has existed the problem that when such smooth surfaces come into contact with each other, an adsorption phenomenon occurred, whereby the head was not operative. Particularly in the CSS system, this problem has occurred with a high probability. Furthermore, there has existed the problem that according to the CSS system, in a disk area wherein a head was put on standby, the surface of the area became coarse, and thus such a disk area could not be recorded, whereby the recording capacity was inevitably decreased. Additionally, according to the CSS system, when a hard disk unit was impacted, the collision of a head and a disk was caused with a high probability, whereby the disk and data tended to be damaged.
In order to solve such problems of a CSS system so as to enhance reliability, recently the CSS system tends to be shifted to a Load/Unload system wherein a component part referred to as a ramp is provided on the outside of a disk in order to house and guide a head within a hard disk drive. In most hard disks as used, in particular for a notebook computer, a car navigation system or a mobile application, a Load/Unload system is employed.
A polyacetal resin is excellent in balance between mechanical strength, chemical resistance and sliding properties, and in working properties, and thus is broadly used as a typical engineering plastic, and mainly for electrical machinery and mechanism components thereof, and automobile components. Furthermore, the extension of its utilized field in recent years has been accompanied with the extensive utilization of various mechanism components. As an example thereof, a ramp within a hard disk drive can be cited.
For example, JP-11-339411A discloses a copolyester of 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid (HAHN), polyacetal, or polyether ether ketone as a ramp material. Furthermore, JP-2001-23325A discloses polyimide, polyacetal, PEEK, HAHN, a liquid crystal polymer, and a PTFE-filled plastic as a ramp-unit material. Besides, JP-10-125014A discloses a synthetic resin such as polyacetal as a ramp material. Furthermore, JP-10-312657A discloses a ramp whose supporting section is formed with a thermoplastic polyimide, whose sliding section is formed with a polytetrafluoroethylene-contained liquid-crystal polymer. Besides, JP-10-302421A discloses that a dry lubricating film (of a tetrafluoroethylene resin, an imide resin, or the like) is formed on the contact area (or sliding surface) of a ramp. Furthermore, each of JP-2002-124051A; 2001-229634A; and 2001-195853A discloses that a polymeric material having a low coefficient of friction, or a resin material having excellent frictional wear properties is applied to the guide area (or sliding portion) of a ramp.
A method using for a ramp a copolyester of 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid (HAHN), polyether ether ketone, polyimide, a liquid crystal polymer, a PTFE-filled plastic, or a polytetrafluoroethylene-contained liquid crystal polymer, as employed in the prior art, had the problem that an abrasion loss at the resin portion of the ramp was remarkably large, said abrasion loss being generated when a hard disk was operated, whereby a metallic tab used as a mating material was scraped therewith. Particles resulting from the abrasion crashed the hard disk and/or gave rise to a disturbance of recording and/or regenerating. Furthermore, the method of forming a dry lubricating film (of a tetrafluoroethylene resin, an imide resin, etc.) on a contact area (or a sliding surface) of a ramp, and the like could not satisfactorily decrease abrasion loss at the resin portion of a ramp, said abrasion loss being generated when a hard disk drive was operated.
On the other hand, JP-2001-297548A discloses a ramp material having a tensile elongation of 30% or more based upon ASTM D638. This prior art discloses a polyacetal ramp, but discloses no polyacetal copolymer having a surface hardness as defined in the present invention and a particular shear stress. Besides, even if the above material having a tensile elongation of 30% or more based upon ASTM D638 is employed, a ramp with all the performance requirements that abrasion loss at the resin portion of a ramp is small (or abrasion loss after a durability test through reciprocating motions is small) when a hard disk drive is operated, and a resin portion small in dimensional change and excellent in moldabilities (with no molding sinks, no casting fins, and a short molding cycle) can not be obtained. Additionally, the sliding properties might be remarkably decreased depending upon molding conditions or the like, and the abrasion loss at the resin portion of the ramp might be increased when a hard disk is operated.
Furthermore, JP-2002-197820A discloses a material for a ramp, comprising a thermoplastic or thermosetting resin with an inorganic filler added in an amount of 4% by weight or less. This prior art discloses a ramp made of polyacetal, but discloses no polyacetal copolymer having a surface hardness as defined in the present invention, and a particular shear stress. Besides, even if such a material comprising a thermoplastic or thermosetting resin with an inorganic filler added in an amount of 4% by weight or less is employed, a ramp with all the performance requirements that abrasion loss at the resin portion of the ramp is small (or abrasion loss after a durability test through reciprocating motions is low) after a hard disk drive is operated, and a resin portion small in dimensional change and excellent in moldabilities (with no molding sinks, no casting fins, and a short molding cycle) could not be derived. Additionally, the sliding properties might be remarkably decreased depending upon molding conditions or the like, and the abrasion loss at the resin portion of the ramp might be increased after the hard disk drive is operated.
It is an object of the present invention to provide a ramp made of a polyacetal resin, which is excellent in moldabilities (with no molding sinks, no casting fins, and a short molding cycle), whose resin portion has a remarkably small abrasion loss after a hard disk drive is operated, which could not be derived in the prior art.